The present invention generally relates to fuel-fired heating appliances, such as forced air furnaces, water heaters and boilers, and more particularly relates to apparatus and methods for reducing the NO.sub.x emissions thereof.
Combustion products formed during the operation of gas-fired heating appliances such as forced air furnaces, water heaters and boilers typically contain NO.sub.x (oxides of nitrogen) which may undesirably be discharged to atmosphere. NO.sub.x is produced by the oxidation of atmospheric nitrogen in high temperature regions of the burner flames. The process is endothermic (i.e., temperature dependent), and typically proceeds at significant rates only at temperatures above about 1800.degree. F.
In forced draft gas-fired heating appliances using shot-type burners, the burner flames (and resultant hot combustion gases) are drawn through heat exchanger combustor tubes at a relatively high velocity by a draft inducer fan which ultimately discharges the spent combustion gases to atmosphere. The medium to be heated by the appliance (air in the case of a furnace, and water in the case of a water heater or boiler) is flowed externally across the heat exchanger structure which transfers combustion heat thereto.
In order to reduce the amount of NO.sub.x formed during the combustion process, the burner flames must be "quenched" (i.e., reduced in temperature). One method of accomplishing this flame quenching is to insert a "heat sink" member into the flame path to absorb a portion of its heat energy, thereby reducing its temperature and corresponding NO.sub.x levels. It is also possible to cool the flame by dispersing it, causing it to lose its "tight" pattern and become somewhat disorganized. The resulting random or unconcentrated flame is cooler than the formed flame, also resulting in lowered NO.sub.x levels.
Conventionally configured thermally conductive heat sink members interposed in the flame path for quenching purposes also tend to disrupt and disperse the flame, thereby lowering the level of NO.sub.x generation by each of these two mechanisms. However, particularly in the case of forced draft heating appliances using shot-type burners in combination with tube-type heat exchanger structures, conventionally configured heat sink members tend to generate excessive a mounts of operational noise while performing their NO.sub.x reducing functions. Forced draft heating appliances provided with these conventionally configured heat sink members thus typically tend to be unsuitable for indoor residential installations where quietness of operation is a primary design consideration.
It can be seen from the foregoing that it would be desirable to provide a NO.sub.x reducing heat sink device that is sufficiently quiet in operation to permit its associated appliance to be used in indoor residential installations. It is accordingly an object of the present invention to provide such a device.